It is well known that modern communications systems must have large bandwidths in order to carry the large amounts of information required by post-industrial information-dependent society. These large bandwidths are most easily accommodated in high-frequency communications systems, such as light or radio-frequency (RF), by which is meant those frequency ranges which include microwave and millimeter-wave. It may be necessary to convert signals carried by or modulated on microwaves or millimeter-waves into signals carried by light, and to similarly convert signals carried by light into signals carried by RF. These conversions are known as modulation or demodulation, depending upon whether a signal is impressed upon a carrier, or a carrier is removed.